System and method for control pf layer formation in an aluminum electrolysis cell

ABSTRACT

An objective of the present invention is to provide a method and system for use for control of layer formation over an extended area in an aluminium electrolysis cell and exploitation of heat. A second object of the invention is to provide a method and system for use for control of layer formation suited for retrofitting to an aluminium electrolysis cell and maintainability during operations of the cell. 
     The present invention attains the above-described objectives by a flat heat tube for attachment to the steel casing of an aluminium electrolysis cell. This heat tube can be a heat pipe or a thermosyphon. Preferably the heat tube is provided with a substantially flat surface. Preferably the heat tube has a meandering shape.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to heat regulation in general and particularly improved method and system for cooling over a large area, suitable for use for control of layer formation over an extended area in an aluminium electrolysis cell and exploitation of heat.

2. Background Art

During production of aluminium with electrolysis technology of today based on so called Hall-Héroult cells, the operations of the cells depend on the formation and maintenance of a protective layer of frozen electrolyte in the side walls of the cell. This frozen bath is called side layer and protects the side lining of the cells against chemical and mechanical wear, and is an essential condition for achieving long lifetime of the cells. The frozen bath operates simultaneously as a buffer for the cell with regards of changes in the heat balance. During operations the heat generation and the heat balance of the cell will vary due to unwanted disturbances of the operation (changes in bath acidity, changes in alumina concentration, changes in interpolar distances, etc.) and desired activities on the cells (metal tapping, change of anode, fire, etc.). This causes the thickness of the layer of the periphery of the cell to change and in some cases the layer will disappear entirely in parts of the periphery. Then the side lining will be exposed against the electrolyte and metal, which in combination with oxidizing gasses will lead to corrosion of the side lining materials causing these to erode. During operations over time run outs in the side can result from such repeated occurrences. It is therefore of importance to control formation of layer and layer stability in Hall-Héroult cells. For Hall-Héroult cells with high current densities model calculations show that it will be difficult to maintain the side layer of the cell due to large heat generation. For such cells and for traditional cells with heat balance problems it will therefore be a condition for a long life cell that one is able to maintain the layer protecting the side lining.

During production of aluminium in accordance with Hall-Héroult principle, this takes place at present with relatively high use of energy as measured in kilo watt hours per kilo aluminium. The heat generation of the electrolysis cells takes place as a result of ohmic voltage drops in the cell, for instance in current feeds, produced metal and particularly in the electrolyte. Approximately 55% of input energy to the electrolysis cell is used for heat generation in the cell. Data from literature indicates that approximately 40% of the total heat loss from the cells is lost through the side lining. Due to the high heat loss and the protecting frozen layer in the side lining it is a preferable place to place elements for heat regeneration in this area of the cell.

There is a desire for optimizing control of layer formation and heat regeneration. In order to optimize both of this purpose at the same time it is important that heat regeneration takes place as close to the formed side layer as possible. This will lead to the control of and speed on layer formation is as fast as possible, and that temperature difference between input and output cooling medium is as large as possible. The latter is preferable for exploitation/regeneration of energy.

Furthermore, due to the large scale of electrolysis cells, it is also desirable to control said layer formation over an extended area since loss of layer formation over a small area can be damaging.

The traditional method of removing heat was to use air convection over the entire surface area of the cell, resulting in limited potential for exploitation of the removed heat.

From the known art one should refer to granted patent NO 318012, corresponding to WO/2004/083489. This describes a sidelining formed with hollows for flow-through of a cooling medium. The manufacturing process of this, however, is complex and requires the side linings to be moulded with hollows formed preferably before the material is sintered.

From the known art one should also refer to patent application NO 20101321, brought into the PCT-phase as PCT/NO2011/000263, of the present applicant. This describes a system for use for control of layer formation in an aluminium electrolysis cell and exploitation of heat comprising sidelining provided with at least one hollow for heat transfer and at least one heat tube, characterized in that the heat tube is provided by the hollow and that the hollow is at least one canal provided along the surface of the sidelining. The manufacturing process of this, however, is complex and requires providing the side linings with a large number of heat tubes, typically heat pipes, along the surface of the sidelining, each requiring separate cooling.

One should also refer to flat heat pipes, also known as two-dimensional heat pipes, based on plates forming thin planar capillaries. This design is useful for heat spreaders in height sensitive applications, however as the capillaries are small and thin the total heat transfer is small. Also this design features large metal areas that are not actual parts of the capillaries, further reducing the total heat transfer.

Furthermore this design is typically flat whereas some surface roughness of a sidelining should be expected, leading to poor thermal contact. This means that flat heat pipes are not suited for cooling a sidelining.

In general it is a problem that efficient cooling over a large area requires a large number of parts which in turn adds complexity and cost while also reduces overall reliability.

There is therefore a need for a method and a system overcoming the above mentioned problems.

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Therefore, a main objective of the present invention is to provide a method and system for use for control of layer formation over an extended area in an aluminium electrolysis cell and exploitation of heat.

A second object of the invention is to provide a method and system for use for control of layer formation suited for retrofitting to an aluminium electrolysis cell.

Means for Solving the Problems

The objective is achieved according to the invention by a system for use for control of layer formation in an aluminium electrolysis cell as defined in the preamble of claim 1, having the features of the characterising portion of claim 1, and a method for control of layer formation in an aluminium electrolysis cell as defined in the preamble of claim 8, having the features of the characterising portion of claim 10. The present invention attains the above-described object by a flat heat tube for attachment to the steel casing of an aluminium electrolysis cell. This heat tube can be a heat pipe or a thermosyphon, or even a cooling pipe using a fluid not undergoing substantial phase transition. Preferably the heat tube is provided with a substantially flat surface. Preferably the heat tube has a meandering shape.

In a first embodiment a system for use for control of layer formation in an aluminium electrolysis cell and exploitation of heat, said electrolysis cell comprising a sidelining and a shell is provided, further comprising a surface attached heat tube provided with means for attachment to said shell.

-   In a preferred embodiment the heat tube is a heat pipe. -   In a another embodiment the heat tube is a thermosyphon. -   In a further embodiment the thermosyphon is provided with a     substantially downward inclination. -   In a preferred embodiment the heat tube has a meandering shape. -   In a preferred embodiment the heat tube is provided with at least     one flat face suited for attachment to the surface of the steel     shell 8 of the electrolysis cell. -   In a embodiment the flat face is provided with a longitudinal track. -   In a preferred embodiment the longitudinal track runs parallel to     the heat tube. -   In a another preferred embodiment the longitudinal track runs in a     meandering path with respect to the heat tube. -   In a preferred embodiment a method for control of layer formation in     an aluminium electrolysis cell said electrolysis cell comprising a     sidelining and a shell is provided, further comprising a heat tube     provided with means for attachment to said shell is attached to said     shell, conducting the heat away using said surface attached heat     tube.

Effects of the Invention

The technical differences over prior art is that the present invention is for attachment to the steel shell covering a ceramic block, and freezes out a sidelayer in the electrolyte bath by extracting heat through the ceramic which is in thermal contact with the steel shell. Prior art in contrast describes cooling by embedding fooling features in or embedded in the ceramic block.

The technical effect of the flat heat tube is that it extracts heat from a large area. Also by using a meandering shape a large surface area can be covered by a single heat tube.

These effects provide in turn several further advantageous effects:

-   -   it makes it possible to provide a convenient solution having few         parts for cooling a large area,     -   it provides a robust and greatly simplified production and         installation without the need of purpose formed ceramic blocks,     -   it can be retrofitted to existing cells, even during         uninterrupted operation     -   reduction of risks in emergency situations     -   maintenance during operation, and     -   the heat pipe can be changed during operation

For heat tube employing substantially phase transition some further advantages are:

-   -   the heat pipe converts between low heat flux in the evaporation         side to high heat flux on the condensation side, enabling a         small size heat exchanger,     -   only a limited amount of working fluid is required, and     -   the surface of the heat pipe is isothermal.

It should be noted that the present invention differs from other heat pipe solutions such as circulating heat pipes where fluid in vapour phase flows in separate tubes from tubes conducting fluid in liquid phase, thus adding extra tubes and further complexity.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described below in connection with exemplary embodiments which are schematically shown in the drawings, wherein:

FIG. 1 shows state of the art of a Hall-Héroult cell in the form of a sidelining block, and a steel shell or casing,

FIG. 2 shows a detail section of the embodiment of FIG. 1 together with section as seen from the side,

FIG. 3 shows state of the art of a Hall-Héroult cell in the form of a sidelining block with hollows provided with heat tube, and a steel shell or casing,

FIG. 4 shows the present invention installed on a cell

FIG. 5 a shows an end view of a first embodiment,

FIG. 5 b shows a side view of the first embodiment,

FIG. 5 c shows a front view of the first embodiment,

FIG. 5 d shows a cross section A-A of the first embodiment,

FIG. 5 e shows a cross section A-A of the first embodiment having a longitudinal track,

FIG. 5 f shows a cross section B-B of the first embodiment,

FIG. 6 a shows an end view of a second embodiment,

FIG. 6 b shows a side view of the second embodiment,

FIG. 6 c shows a front view of the second embodiment,

FIG. 6 d shows a cross section A-A of the second embodiment, and

FIG. 6 e shows a cross section A-A of the second embodiment having a longitudinal track.

DESCRIPTION OF THE REFERENCE SIGNS

The following reference numbers and signs refer to the drawings:

1 Anode hanger 2 Anode carbon block 3 Liquid electrolyte 4 Liquid aluminium 5 Cathode carbon 6 Frozen electrolyte 7 Insulating brickwork 8 Steel shell 9 Ramming paste 10 Heat insulation 11 Sidelining block 12 Heat tube 13 Condensation unit for heat tube 14 Condensation fins 15 Thermal paste between sidelining block and steel shell 100 Surface mounted heat tube 110 Heat tube cold end 120 Condensation unit for heat tube 122 Condensation fins 124 Fluid connector 130 Heat tube hot end 132 Lower end of heat tube 140 Flat face 142 Flat face attachment holes 144 Transverse member 146 Transverse member attachment holes 150 Longitudinal track 152 Entry hole to longitudinal track 154 Exit hole to longitudinal track

DETAILED DESCRIPTION

The invention will in the following be described in more details with references to the drawings showing embodiments and where FIG. 1 shows state of the art of a Hall-Héroult cell in the form of a sidelining block 11 and a steel shell 8 or casing. Details are shown in FIG. 2. A state of the art cell using active cooling as known from previously mentioned prior art is shown in FIG. 3.

With sidelining one should here understand this to mean sidelining block 11, optionally in the case of state of the art together with the heat insulation 10, wherein the sidelining block is optionally provided with heat tube 12. The sidelining block 11 is typically a ceramic block, typically in the form of silicon carbide (SiC).

Principles Forming the Basis of the Invention

By heat tube 12, 100 there are two embodiments intended: “heat pipe” where a wick or other capillary effect pulls the liquid back to the hot end, and “thermosyphon” where the gravity pulls the liquid back to the hot end. The hot end is also known as the evaporation section. Both principles can be applied for this invention, though a thermosyphon it is preferred that the tube body is provided with a substantially downward inclination so that fluid in the liquid phase can run down the length of the tube. Since heat tubes of either type operate by removing heat by phase transition liquid to gas, it is preferred that the heat tube allows liquid to reach the lowest point in the heat tube.

A typical Hall-Héroult cell comprises a steel casing or shell 8, surrounding a sidelining block 11. The steel casing is in good thermal contact with sidelining block due to a thermal paste. The sidelining block, on the opposite side from the steel casing, is in contact with the electrolyte containing aluminium (Al). By use of thermal control the heat extracted from the electrolyte builds up a layer of frozen electrolyte on the sidelining, leaving the remaining part of the electrolyte 3 in the liquid phase.

Central in the invention is the realisation that it is possible to remove a sufficient amount of heat by attaching a heat tube 100 to the steel casing. When a heat tube operates to remove heat, the steel shell does not become overheated, and with the high thermal conductivity present through the metal and the thermal paste, the layer of frozen electrolyte 6 can be maintained.

Best Modes of Carrying Out the Invention

The embodiment of the apparatus according to the best mode of invention shown in FIGS. 5 and 6 comprises a meandering thermosyphon 100 hot end having a substantially continuously downward component with reference to gravity, and an alternating right and left component horizontally. At the upper end a cold end is attached. The cold end is also known as the condensation end. At the cold end the condensation unit is placed having condensation fins attached to the cold end for efficient heat transfer. Fluid enters a first fluid connector, into the condensation unit where heat is removed from the cold end, and out through a second fluid connector.

As working fluid in the liquid phase enters the hot end 130 the fluid runs downwards, evaporating as it flows and thus removes heat from the evaporator end. It is preferred that a sufficient amount of heat reaches the lower end 132 of the heat tube 100 in order to extract heat along the entire length of the heat tube from the cell through the sidelining block 11.

Some horizontal extents of the heat tube can be accepted, also small amounts of depressions. Any depressions will catch fluid in the liquid state, limiting the amount that continues downward.

As heat is absorbed by the fluid in the liquid state it will make a phase transition to the vapour phase, boiling, and the fluid in the vapour phase will move upwards to the top where the cold end 110 or condensation end is positioned. Condensation to the liquid phase takes place using a heat exchanger 130, transferring the heat typically to an oil or molten salt circuit.

The heat tube 100 is typically in the form of a thermosyphon, since gravity is sufficient for ensuring fluid in the liquid phase is transported down the heat tube.

In a preferred embodiment the heat tube is provided with at least one flat surface suited for attachment to the surface of the steel shell 8 of the electrolysis cell. This can be made through moulding as well as by welding a traditional heat tube to a suitably formed part having a flat surface or flat face 140. For the attachment it is preferred that the surface attached heat tube is provided with holes 142 for attachment to the steel shell. Holes can be provided along the sides of the flat face of the heat tube. The flat surface is then provided with thermal conductive paste, of which many are well known in the art, and then attached to the steel shell using the holes by bolts, screws or other methods well known in the art.

For improving efficiency of extraction of useful heat the steel shell assembly with the heat tube is provided with thermal insulation, preventing heat from leaking into the surroundings. This is shown in FIG. 4.

Alternative Embodiments

A number of variations on the above can be envisaged. For instance one can envisage a hybrid heat pipe—thermosyphon solution, wherein a wick structure raises the liquid phase through capillary action up along the part of the tube close to the external flat face of the heat tube, thus increasing the effective area for phase transition.

Several variations can be envisaged for attachment holes. In addition to the holes described above, one can also provide the surface attached heat tube with transverse members 144, extending substantially transverse to the heat tubes. Said members provide a better working distance for providing holes 146 and attachment, with respect to the more fragile heat tubes 12.

Providing sufficient and even distribution of heat paste is important for optimal heat transfer. While manual application of thermal paste prior to attachment is a simple solution it is not certain to be sufficient. First of all the outer surface of the steel shell 8 might not be as smooth and level as the flat face of the heat tube. Furthermore it might become necessary later to add more or replenish lost heat paste. One solution is to provide a longitudinal track 150 in the flat face and preferably also an access hole to 152 this track for applying heat paste under pressure through said hole and into said track, preferably until heat paste starts exiting through an exit hole 154. The longitudinal track can be straight or meandering. The effect of this is to simplify application of heat paste and enable replenishing of heat paste without having to interrupt operations of the cell. Alternatively adhesive or thermal glue can be used.

Also the steel shell and the heat tubes might not be as separate parts, rather the heat tubes could be moulded into the steel shell during manufacture, as a monolithic unit.

While the shell of the cell is described as being made of steel, it should be clear that any other material will also work as long as it can conduct heat and withstand the temperatures involved. Several alternatives can be envisaged, such as ceramic materials.

Industrial Applicability

The invention according to the application finds use in control of layer formation in an aluminium electrolysis cell and exploitation of the heat. 

1.-10. (canceled)
 11. A system for use for control of layer formation in an aluminium electrolysis cell and exploitation of heat, said electrolysis cell comprising a sidelining (11) and a shell (8); wherein a surface attached heat tube (100) is provided with means for attachment (142, 144, 146) to said shell.
 12. The system according to claim 11, wherein the heat tube is a heat pipe.
 13. The system according to claim 11, wherein the heat tube is a thermosyphon.
 14. The system according to claim 13, wherein the thermosyphon is provided with a substantially downward inclination.
 15. The system according to claim 11, wherein the heat tube has a meandering shape.
 16. The system according to claim 11, wherein the heat tube is provided with at least one flat face (140) suited for attachment to the surface of the steel shell 8 of the electrolysis cell.
 17. The system according to claim 16, wherein the flat face (140) is provided with a longitudinal track (150).
 18. The system according to claim 17, wherein the longitudinal track runs parallel to the heat tube.
 19. The system according to claim 17, wherein the longitudinal track runs in a meandering path with respect to the heat tube.
 20. A method for control of layer formation in an aluminium electrolysis cell said electrolysis cell comprising a sidelining (11) and a shell (8); wherein a heat tube (12) provided with means for attachment to said shell is attached to said shell, wherein heat is conducted away using said surface attached heat tube. 